Aromatic dicarboxylic acids are commercially important as raw materials in the manufacture of high molecular weight polymers, such as polyethylene terephthalate, which is used in making synthetic fibers, films and the like products. Methods for manufacturing aromatic carboxylic acids are numerous. Methods for producing aromatic dicarboxylic acids are disclosed in U.S. Pat. No. 4,855,492 to Hundley et al. and U.S. Pat. Nos. 4,709,080 and 4,772,742 to Spanswick et al. Another method specifically for synthesizing terephthalic acid is disclosed in U.S. Pat. No. 4,185,073 to Marsh et al.
In order to be useful as a raw material for polymer manufacture, the aromatic dicarboxylic acid monomer must have relatively high purity prior to polymerization. Unfortunately, conventional processes often produce such carboxylic acid monomers with excessive amounts of colored and color-forming impurities as well as metal compounds that must be reduced.
Most aromatic dicarboxylic acids are produced by the oxidation of alkyl aromatic compounds. Impurities, such as, for example, aromatic aldehyde and ketone impurities, result from incomplete oxidation of those compounds and must also be removed from the resultant aromatic dicarboxylic acids. Unfortunately, because of the varying characteristics of the impurities present, solvents that will remove the aromatic aldehyde and ketone impurities do not necessarily effectively remove the color-forming or metal impurities that are present as well. Also, purification processes that effectively purify other materials are ineffective for purifying aromatic dicarboxylic acids because of the insolubility of these aromatic acids in conventional solvents.
Crystallization techniques, such as the one disclosed in British Patent No. 1,271,779 to Witt, have been employed to remove aromatic aldehydes and ketones as well as color-forming and metal impurities from aromatic dicarboxylic acids. The crystallization technique disclosed in Witt, however, requires rather severe processing conditions, i.e., temperatures of 300.degree. to 325.degree. C. Furthermore, the aromatic dicarboxylic acid is purified in a solution of phenol and water. Phenol is a toxic substance, and employee exposure to this material should be minimized. Finally, the phenol esterifies the carboxylic acid, resulting in the loss of a portion of the desired product. These factors make the Witt recrystallization process undesirable.
Another process utilized to purify carboxylic acids requires the conversion of the acids to dimethyl esters, which are reasonably soluble in hydrocarbons and which have sufficiently high vapor pressures to permit distillation of the resulting mixtures. This conversion/purification technique is costly and time consuming as well.
Prior art processes specific for the purification of terephthalic acid are described in U.S. Pat. No. 3,574,727 to Taylor et al. and U.S. Pat. No. 3,592,847 to Gallivan et al. The Taylor et al. process consists of dissolving the terephthalic acid in a mixture of lower alkanoic acid and lower alkanoic anhydride and recrystallizing the purified terephthalic acid from the solution. The Gallivan et al. process purifies terephthalic acid by dissolving 5 to 20 weight percent of the acid in a heated solution of acetic anhydride to produce a super-saturated solution. Any excess crude terephthalic acid which may have been added to the anhydride and remains undissolved in the solution is filtered from the solution, preferably before the addition of a lower alkanoic acid (e.g. acetic acid), to oxidize the resultant terephthalic acid is then recovered by recrystallization.